It is conventionally known that holographic optical traps may be used for manipulating small objects dispersed in fluid. For example, material transport can result from a process resembling peristaltic pumping, with a sequence of holographically-defined manifolds of traps resembling the states of a physical peristaltic pump. In all such conventional approaches, transport transverse to the optical axis cannot be accomplished by a single optical intensity pattern, but rather requires three or more separate patterns of optical traps to be projected. Other approaches based on generalized optical traps (commonly known as optical vortices) trap small objects on rings of light transverse to the optical axis and drive them in circular orbits around these rings. The resulting motion can be used to drive fluid flow and thus transport other objects in the plane transverse to the optical axis. Such indirect optical driving requires conditions conducive to the operation of optical vortices, sufficiently open geometry and a sufficiently low flow rate.
In order to best understand the improvements of the invention, FIG. 1 illustrates prior art methods and systems which can be used to advantage in the invention features of characterizing and removing optical aberrations. In the prior art optical tweezer system 10 of FIG. 1, optical gradient forces exerted by a single beam of light 12 are used to controllably manipulate a small dielectric particle 14 dispersed in a medium 16 whose index of refraction, nm, is smaller than that of the particle 14 at optical frequencies. The nature of the optical gradient forces is well known, and also it is well understood that the principle has been generalized to allow manipulation of reflecting, absorbing and low dielectric constant particles as well. Any of these techniques can be implemented in the context of the invention described hereinafter and will be encompassed by use of the terminology optical tweezer, optical trap and optical gradient force trap hereinafter.
The optical tweezer system 10 is applied by using a light beam 12 (such as a laser beam) capable of applying the necessary forces needed to carry out the optical trapping effect needed to manipulate a particle. The object of a conventional form of the optical tweezer 10 is to project one or more beams of light into the center of a back aperture 24 of a converging optical element (such as an objective lens 20). As noted in FIG. 1, the light beam 12 has a width, w, and has an input angle φ relative to an optical axis 22. The light beam 12 is input to a back aperture 24 of the objective lens 20 and output from a front aperture 26 substantially converging to a focal point 28 in focal plane 30 of imaging volume 32 with the focal point 28 in near association with an optical trap 33. In general, any focusing optical system can form the basis for the optical tweezer system 10.
In the case of the light beam 12 being a collimated laser beam and having its axis coincident with the optical axis 22, the light beam 12 enters the back aperture 24 of the objective lens 20 and is brought to a focus in the imaging volume 32 at the center point c of the objective lens' focal plane 30. When the axis of the light beam 12 is displaced by the angle φ with respect to the optical axis 22, beam axis 31 and the optical axis 22 coincide at the center point B of the back aperture 12. This displacement enables translation of the optical trap across the field of view by an amount that depends on the angular magnification of the objective lens 20. The two variables, angular displacement φ and varying convergence of the light beam 12, can be used to form the optical trap at selected positions within the imaging volume 32. A plurality of the optical traps 33 can be arranged in different locations provided that multiple beams of light 12 are applied to the back aperture 24 at different angles φ and with differing degrees of collimation.
In order to carry out optical trapping in three dimensions, optical gradient forces created on the particle to be trapped must exceed other radiation pressures arising from light scattering and absorption. In general, this necessitates having the wave front of the light beam 12 to have an appropriate shape at the back aperture 24. For example, for a Gaussian TEM00 input laser beam, the beam diameter w should substantially coincide with the diameter of the back aperture 24. For more general beam profiles (such as Laguerre-Gaussian modes) comparable conditions can be formulated.
In another prior art system in FIG. 2, the optical tweezer system 10 can translate the optical trap 33 across the field of view of the objective lens 20. A telescope 34, or other relay optics, is constructed of lenses L1 and L2 which establishes a point A which is optically conjugate to the center point B in the prior art system of FIG. 1. In other forms of the invention the relay optics can include other conventional systems, such as multiple optical elements to minimize aberrations. In the system of FIG. 2 the light beam 12 passing through the point A also passes through the point B and thus meets the basic requirements for performing as the optical tweezer system 10. The degree of collimation is preserved by positioning the lenses L1 and L2 as shown in FIG. 2. The transfer properties of the telescope 34 can be chosen to optimize angular displacement of the light beam 12 and its width w in the plane of the back aperture 24 of the objective lens 20. As stated hereinbefore, in general several of the light beams 12 can be used to form several associated optical traps. Such multiple beams 12 can be created from multiple independent input beams or from a single beam manipulated by conventional reflective and/or refractive optical elements.
In another prior art system shown in FIG. 3, arbitrary arrays of optical traps can be formed. A diffractive optical element 40 is disposed substantially in a plane 42 conjugate to back aperture 24 of the objective lens 20. Note that only a single diffracted output beam 44 is shown for clarity, but it should be understood that a plurality of such beams 44 can be created by the diffractive optical element 40. The input light beam 12 incident on the diffractive optical element 40 is split into a pattern of the output beams 44 characteristic of the nature of the diffractive optical element 40, each of which emanates from the point A. Thus the output beams 44 also pass through the point B as a consequence of the downstream optical elements described hereinbefore.
The prior art diffractive optical element 40 of FIG. 3 is shown as being normal to the input light beam 12, but many other arrangements are possible. For example, in the prior art system of FIG. 4 the light beam 12 arrives at an oblique angle β relative to the optic axis 22 and not at a normal to the diffractive optical element 40. In this embodiment, the diffracted beams 44 emanating from point A will form optical traps 50 in focal plane 52 of the imaging volume 32 (seen best in FIG. 1). In this arrangement of the optical tweezer system 10 an undiffracted portion 54 of the input light beam 12 can be removed from the optical tweezer system 10. This configuration thus enables processing less background light and improves efficiency and effectiveness of forming optical traps.
The diffractive optical element 40 can include computer generated holograms which split the input light beam 12 into a preselected desired pattern. Combining such holograms with the remainder of the optical elements in FIGS. 3 and 4 enables creation of arbitrary arrays in which the diffractive optical element 40 is used to shape the wavefront of each diffracted beam independently. Therefore, the optical traps 50 can be disposed not only in the focal plane 52 to form a three-dimensional arrangement of the optical traps 50.
In the optical tweezer system 10 of FIGS. 3 and 4, also included is a focusing optical element, such as the objective lens 20 (or other like functionally equivalent optical device, such as a Fresnel lens) to converge the diffracted beam 44 to form the optical traps 50. Further, the telescope 34, or other equivalent transfer optics, creates a point A conjugate to the center point B of the previous back aperture 24. The diffractive optical element 40 is placed in a plane containing point A.
In another form of prior art system, arbitrary arrays of the optical traps 50 can be created without use of the telescope 34. In such an embodiment the diffractive optical element 40 can be placed directly in the plane containing point B.
In the optical tweezer system 10, either static or time dependent diffractive optical elements 40 can be used. For a dynamic, or time dependent version, one can create time changing arrays of the optical traps 50 which can be part of a system utilizing such a feature. In addition, these dynamic optical elements 40 can be used to actively move particles and matrix media relative to one another. For example, the diffractive optical element 40 can be a liquid crystal phase modulating array that imprints computer generated holographic patterns onto incident light.
In another prior art system illustrated in FIG. 5, a system can be constructed to carry out continuous translation of the optical tweezer trap 50. A gimbal mounted mirror 60 is placed with its center of rotation at point A. The light beam 12 is incident on the surface of the mirror 60 and has its axis passing through point A and will be projected to the back aperture 24. Tilting of the mirror 60 causes a change of the angle of incidence of the light beam 12 relative to the mirror 60, and this feature can be used to translate the resulting optical trap 50. A second telescope 62 is formed from lenses L3 and L4 which creates a point A′ which is conjugate to point A. The diffractive optical element 40 placed at point A′ now creates a pattern of diffracted beams 64, each of which passes through point A to form one of the tweezer traps 50 in an array of the optical tweezers system 10.
In operation of the embodiment of FIG. 5, the mirror 60 translates the entire tweezer array as a unit. This methodology is useful for precisely aligning the optical tweezer array with a stationary substrate, for dynamically stiffening the optical trap 50 through small-amplitude rapid oscillatory displacements, as well as for any applications requiring a general translation capability.
The array of the optical traps 50 also can be translated vertically relative to the sample stage (not shown) by moving the sample stage or by adjusting the telescope 34. In addition, the optical tweezer array can also be translated laterally relative to the sample by moving the sample stage. This feature would be particularly useful for large scale movement beyond the range of the objective lens' field of view.
In another prior art system shown in FIG. 6, the optical system is arranged to permit viewing images of particles trapped by the optical tweezers 10. A dichroic beamsplitter 70, or other equivalent optical beamsplitter, is inserted between the objective lens 20 and the optical train of the optical tweezer system 10. In the illustrated embodiment the beamsplitter 70 selectively reflects the wavelength of light used to form the optical tweezer array and transmits other wavelengths. Thus, the light beam 12 used to form the optical traps 50 is transmitted to the back aperture 24 with high efficiency while light beam 66 used to form images can pass through to imaging optics (not shown).
A prior art application of optical traps is shown in FIGS. 7A and 7B. The diffractive optical element 40 is designed to interact with the single light beam 12 to create a 4×4 array of collimated beams. A 100 mW frequency doubled diode-pumped Nd:YAG laser operating at 532 nm provides a Gaussian TEM00 form for the light beam 12. In FIG. 7A the field of view is illuminated in part by laser light backscattered by sixteen silica spheres trapped in the array's sixteen primary optical tweezers 10. The 1 μm diameter spheres are dispersed in water and placed in a sample volume between a glass microscope slide and a 170 μm thick glass coverslip. The tweezer array is projected upward through the coverslip and is positioned in a plane 8 μm above the coverslip and more than 20 μm below the upper microscope slide. The silica spheres are stably trapped in three dimensions, each in one of the sixteen optical tweezers 10.
In FIG. 7B is shown the optically-organized arrangement of spheres 1/30 second after the optical tweezers 10 (traps) were extinguished but before the spheres had time to diffuse away from the trap site.
Consequently, optical tweezers and related optical traps use forces exerted by the intensity gradients in tightly focused beams of light to trap, move and otherwise modify small volumes of matter in three dimensions. Imprecise alignment and imperfect characteristics of the optical elements of an optical trapping system introduce aberrations into the trapping beam, diminish its intensity gradients, and thereby degrade its ability to manipulate matter. In common practice, the optical elements in optical trapping systems are aligned by systematically adjusting each element's position while observing the apparent quality of the focused optical trap using a optical imaging system. A well aligned optical tweezer comes to a tight and symmetric focus and spreads uniformly and symmetrically when defocused. While simple and reasonably effective, this approach does not generally achieve optimal performance, nor does it provide a quantitative assessment of the optical train's alignment.